I. Technical Field
The present invention concerns a circuit carrier having a metallic carrier layer on which a dielectric layer is arranged in at least a region-wise manner, wherein the dielectric layer has a plurality of pores.
Circuit carriers of that kind are used primarily in the field of power electronics, in particular in the field of high temperature uses, as in the engine compartment of motor vehicles.
II. Description of the Related Art
The operation of any electrical or electronic circuit is linked to the generation of waste heat which, to avoid adversely affecting operation or seriously damaging the circuit, must be dissipated as quickly as possible. In particular power electronics are distinguished in operation by a very large amount of waste heat. As a further difficulty in high temperature uses there is also the relatively high ambient temperature of over 100° C. (for example about 150° C. in certain regions of the engine compartment of motor vehicles). To provide for dissipation of the waste heat as quickly as possible, the electrical or electronic circuits are mounted on circuit carriers, the metallic carrier material of which can function as a cooling body for the circuit.
The layer of dielectric material which is generally very much thinner than that of metallic material serves in particular to electrically insulate the individual conductor tracks arranged on the layer of dielectric material, from the metallic carrier material. In addition it is possible to select a dielectric material which is distinguished by a very low heat transmission resistance so that the waste heat from electrical or electronic components arranged on the layer of dielectric material can be carried away by way of the metallic carrier element as quickly as possible.
After its manufacture the dielectric layer may have a large number of pores. For various reasons it may be necessary to seal those pores with a sealing material. For example, the presence of pores adversely affects the electrical insulation capability of the layer of dielectric material, which can be problematical in particular in a humid working environment. Due to the penetration of moisture into the pores, electrical short-circuits can occur between the conductor tracks and the metallic carrier material.
Depending on the respective production process involved, the problem of the formation of pores occurs to differing degrees. In particular production processes in which the layer of dielectric material is applied to the metallic carrier material in a thermal spray process suffer from the problem of pore formation.
Processes of that kind are described for example in GB 990 023, GB 1 461 031 and EP 115 412 A2. Although the thermal spraying process in itself is highly suitable for applying the dielectric material to the metallic carrier material, there is nonetheless the problem that the sprayed-on layer of dielectric material has a large number of pores, which can considerably reduce the electrical insulation capability of the layer of dielectric material. The presence of pores is particularly problematical in a damp environment.
By way of example, EP 48 992 A2 describes a process in which after thermal spraying of the layer of dielectric material, a resin is spread on to the layer of dielectric material to seal the pores. DE 195 29 627 C2 also discloses sealing the pores by applying an epoxy resin. In addition, sealing of the pores by a ceramic glaze melting in the temperature range of between 600° C. and 800° C. is also described.
The use of resins is disadvantageous insofar as this involves a relatively complicated and expensive process as the resins have to be hardened in a suitable mold (for example by polymerisation—see EP 48 992 A2). The application of a ceramic glaze as additionally described in DE 195 29 627 C1 is disadvantageous insofar as the ceramic glaze which melts at the specified temperatures and which comes from the porcelain industry is heavily lead-bearing, and therefore can no longer be used in most countries. In addition, it has been found that such a ceramic glaze is frequently itself porous so that the film applied to the layer of dielectric material can itself have holes. In that case the problem of the occurrence of short-circuits is not overcome at all.